As the advantages of fiber optic based communication and control of industrial processes becomes better known, increasing emphasis is being placed on various methods of simple, inexpensive, and reliable communication of low level radiant energy via fiber optics to the sensor site for making a desired measurement, and returning the measurement information on fiber optic paths to the control and measurement location. Among the many problems facing designers of such process control systems are the need for few, low light level optical paths and methods of accurately and reliably carrying out the measurements in such a way that the derived measurement information may be accurately communicated by means of fiber optic signals. In the application of resonant element sensors, it is especially important that low power, highly efficient sensors be developed to modulate the light available. One problem is in achieving high opto-mechanical loop gain in order to reduce the optical energy threshold to reasonable levels.
Instruments are well known wherein the resonant frequency of a resonant element subjected to a force is a function of the tension (or compression) applied to that resonator. It has been recognized that a force measuring instrument can be based on this relationship by causing the resonator to vibrate while a tension or compression force is applied thereto and measuring the vibration frequency. An application of this principle for vibrating wire resonators is known from U.S. Pat. No. 4,329,775. The present invention solves a problem presented in efficiently converting optical power to both drive and sense light beam signals for use with resonant elements, not limited to vibration wire resonators.
For the purpose of this limited description, "resonant mechanical structure", "resonator", and "resonant element" generally refer to beam (hollow beam, cantilevered beam and cantilevered hollow beam, and double- or other multiple-beam elements), and ribbon, wire or other articles of manufacture, and their equivalents, all of which can be resonated at particular oscillation frequencies. Specifically includes are tuning fork structures of the single- and double-ended varieties, as well as multiple tine tuning fork structures.
"Fiber optic", "optical fiber", and "radiant energy" path or pathway means and equivalent terms refer to single or multiple communication paths.
As used herein, the term "radiant power", light, optical power or light flux includes electromagnetic power of wavelengths between 0.1 and 100 micrometers, and specifically includes infrared, ultraviolet, and visible light. Here, light flux refers to the number of photons that pass through a plane per unit of time, and is measured in watts. For simplicity, such radiant energy may be referred to generally and without limitation as "light" or "optical" power. Such radiant power may also be described as "steady" or "continuous" or "unmodulated" in order to distinguish it from radiant power signals which are modified to carry information. The term "radiant power" specifically includes coherent and incoherent light power.
"Modulation" is used broadly herein, and it is intended to means modifying (or the modification of) some characteristic or characteristics of a light beam so that it varies in step with the instantaneous value of another signal, and specifically may be used herein to describe amplitude modulation and frequency modulation. "Unmodulated optical power" refers to optical power which is unmodulated in this sense.
"Monochromatic" refers to radiant power composed of a single wavelength. "Collimated light" refers to radiant power having rays which are rendered substantially parallel to a certain line or direction.
"Fluid" includes gases and/or liquids. The term "force" is used to describe any physical parameter or phenomenon capable of moving a body or modifying its motion, and specifically includes force exerted per unit area (pressure) and any parameter or phenomenon capable of conversion to pressure.
"Photothermal effect" and "photokinetic effect", as used herein, refer to the phenomenon wherein photons striking a suitable surface or surface coating cause localized heating, such heating being sufficient to cause localized expansion of the coating or substrate, and thus producing motion.